Generally, in traditional cellular networks, the point where a mobile device attaches and where mobility is controlled is a mobility control node of the core network. As an example, in a Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) compliant network, the node is referred to as a mobility management entity (MME). Each mobility control node has control of the mobile devices within a particular domain, as well as the responsibility for locating the mobile devices and establishing connectivity when communications is needed, such as paging the mobile devices when data service with the mobile device is needed.
Because the mobility control nodes belong to a particular core network, they are intrinsically coupled to the radio access technology (RAT) or technologies (RATs) used in radio access networks (RANs) coupled to that core network. Traditionally, these coupling relationships between RANs and core network nodes have been RAT-specific, with a core network having only one RAT (or at most, a small number of closely related RATs) with which it can communicate. As an example, a MME is specific to mobile devices using 3GPP LTE access. Communicating with devices using other access technologies requires special purpose interworking procedures between the respective core networks, and in general, a particular mobile device would still be associated with one radio access technology or another. For example, a 3GPP LTE network may communicate with a CDMA2000 network to perform an inter-RAT handover, but after the handover, the 3GPP LTE MME considers that the mobile device is no longer under the coverage of the 3GPP LTE MME.
In future networks, using software defined network (SDN) architecture and/or virtualized network functions, the mobility control node will be replaced by a logical function, typically hosted in an Internet protocol (IP) cloud. The virtualized function has no special reason to communicate with only one RAT, and it is reasonable that a single mobility control function may manage devices using multiple RATs.